Capacitive sensors can provide a change in capacitance that represents the proximity of a sensed object. As but one very particular example, capacitive sensors can detect the presence of a pointing object, such as a finger, stylus, or pen. In such an application, an array of capacitive sensors can be scanned, and thereby serve as a pointing device in an electronic system (e.g., portable music player, personal digital assistant, tablet personal computer (PC), notebook PC, desktop PC, to name but a few). Capacitive sensors can be more robust and/or compact than other types of sensors that may include moving parts or more complex position detection approaches.
As understood from above, capacitive sensors can be used in portable devices, such as those operating off of a battery power supply. Reducing power consumption in such portable devices remains an ongoing goal. Accordingly, any reduction in the current draw of a capacitive sensing arrangement would be a welcome step in furthering the battery life of the corresponding device.
To better understand various features of the disclosed embodiment, a conventional capacitive sensing arrangement will now be described.
A conventional capacitive sensing system is set forth in FIG. 7 and designated by the general reference character 700. A system 700 can include an integrated circuit device 702 and a number of capacitive sensors 7041 to 704-N. Each capacitive sensor (704-1 to 704-N) can present a capacitance that can vary according to an input event. For example, a capacitive sensor (704-1 to 704-N) can include an electrode arranged within an array of like electrodes.
An integrated circuit device 702 can include a number of inputs 706-1 to 706-N, switching devices 710-1 to 710-N, a controller section 712, and a sense section 714. Each input (706-1 to 706-N) can be connected to a capacitive sensor (704-1 to 704-N) by a corresponding switching device (710-1 to 710-N). Each switching device (710-1 to 710-N) can be controlled according to a corresponding switching signal SW1 to SWN, and can connect a corresponding input (706-1 to 706-N) to a common node 708.
A controller section 712 can activate individual switching signals to thereby allow scanning of inputs (706-1 to 706-N). A sensing section 714 can determine when the capacitance at a scanned input increases due to an input event.
The general operation of the system of FIG. 7 is shown in a flow diagram in FIG. 8, and designated by the general reference character 800. A method 800 can include selecting a sensor (step 802). Once selected, a charge and dump (step 804) can occur at the resulting capacitor to generate a value representing the sensed capacitance. More particularly, one capacitor can be charged and then connected in parallel to a second capacitor to share accumulated charge. If the value does not reach a threshold (N from 806), a method can return to 804, to repeat that step until the threshold is reached. Once the threshold is reached (Y from 806), a determination can be made on whether an input event has occurred (step 807) (e.g., by measuring the number of times step 804 was repeated to reach the threshold). After this determination, the method can select a next electrode (step 808).
In this way, a set of capacitive sensors can be individually scanned, one-by-one to determine if a change of capacitance has occurred.
While the above conventional approach can provide a suitable method for detecting capacitive sensors of an input device, as noted above, it remains a continuing goal to arrive at some way of reducing the power consumption presented by such arrangements.
One approach to reducing power in a capacitive sensing arrangement like that of FIG. 7 is shown in FIG. 9. FIG. 9 shows a portion of a sensing section, like that shown as 714 in FIG. 7. A sensing section 900 can include two selectable capacitors CA and CS coupled to a common node 908. Capacitor CA can have a greater capacitance than capacitor CS. In an active mode, capacitor CA can be coupled to node 908 while capacitor CS can be isolated from node 908. Thus capacitor CA can serve as an integrating capacitor used to detect a change in capacitance.
In contrast, in a standby mode, capacitor CS can be coupled to node 908 while capacitor CA can be isolated from node 908. Thus, smaller capacitor CS can serve as an integrating capacitor. Such smaller integrating capacitor CS can enable a faster scan of all sensors, and thus reduce power when such scans are performed periodically.
A drawback to an arrangement like that of FIG. 9 can be the increase in circuit components of the resulting system, as well as the need for an extra input to accommodate the second capacitor.
In light of the above, it would be desirable to arrive at some way of reducing power consumption and/or increasing the scanning speed in a capacitive sensing system that does not suffer from the drawbacks of the above conventional approaches.